In recent years, there has been a rapid growth in the amount of information to be processed by electronic devices such as personal computers. As a result, a multilayer wiring board or a semiconductor, which plays an important role in the devices, has increased its density. Further, the size of an electronic member has increased. Under such circumstances, the amount of heat generated inside the device has become larger than that of conventional devices, thereby causing problems such as miscalculation or loss of control of a computer during operation.
In order to solve the problems as set forth above, electronic device manufactures have been taking actions, with the aim of achieving rapid cooling the interior of an electronic device, such as: (1) positioning a cooling fan having higher performances than ever; (2) increasing heat releasing properties of a heat source by positioning, between the heat source and the heat releasing material, a thermal conductive material that is highly heat-conductive and flexible. However, employing a high-performance cooling fan causes an increase in size of an electronic device, and also an increase in noise and cost.
In view of the above, a heat transfer material, which is provided between a heat source and a heat releasing material, has been attracting attention. A heat transfer material is required that it has a thickness of as small as possible, exhibits excellent heat conductivity, and that it can follow deformation of the heat source or the heat releasing material.
Representative examples of the heat transfer material include grease-type materials. However, the method affects workability during assembling of a heat releasing device due to its poor handleability that is attributed to low viscosity. Further, since a grease generally greatly changes its viscosity due to changes in temperature, there is a problem in terms of changes in heat transfer characteristics.
Therefore, a thermal conductive sheet, which has elasticity and is stable in surface tackiness, has been attracting attention as a material that has improved workability compared with grease-type heat transfer materials.
However, since thermal conductive sheets that are currently available in the market are inferior in the balance of heat releasing performances (effect of cooling a heat source), handleability and sheet properties (elasticity and surface tackiness), improvement in these properties has been desired. Further, although a sheet made of indium has been available as a heat releasing metallic material, the use thereof is limited due to its high cost.
Under such circumstances, various kinds of composite materials and processed materials thereof, which are low in cost and exhibit excellent heat releasing properties and workability, have been proposed.
For example, a molded product of a heat releasing resin in which a graphite powder is mixed with a thermoplastic resin (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 62-131033) and a polyester resin composition including graphite, carbon black or the like (see, for example, JP-A No. 4-246456) have been disclosed. Further, a rubber composition in which artificial graphite having a particle size of from 1 to 20 μm is mixed (see, for example, JP-A No. 5-247268) and a composition in which a powder of spheroidal graphite having a crystal surface interval of from 0.330 to 0.340 nm is mixed with a silicone rubber (see JP-A No. 10-298433) have been disclosed.
In addition, a highly heat-releasable composite material in which specific graphite particles are aligned in parallel to the surface of a composition by compressing by applying a pressure in a solid, and a method of producing the material (see, for example, JP-A No. 11-001621) have been disclosed. Further, a heat-releasable molded material in which the c-axis of the crystal structure of a graphite powder in the molded material is oriented in a direction orthogonal to a direction in which heat is released, and a method of producing the molded material has been disclosed (see, for example, JP-A No. 2003-321554).
A thermal conductive sheet that can be readily handled has a feature that assembling of a heat releasing device is easily carried out. In a method of using a thermal conductive sheet in which this feature is further utilized, there has been demand for the thermal conductive sheet to acquire functions such as a shape processability in accord with the shape of a heat releasing material, an ability of following a specific shape such as a rough or warpage of a heat source or a heat releasing material, and a stress relaxation. For example, in a case of releasing heat from a large area, such as a display panel, exhibition of such functions has been an important issue. Accordingly, development of a sheet that can be readily handled and shape-processed, and is highly heat conductive, has been strongly demanded.